Actually, this is a subtle example of Spock prefering to spare Jim’s feelings. The “young lady” Jim is captivated with is actually an android and Spock totally knows this at this point. He could have easily slammed the breaks on Jim’s infatuation by telling him he’s in love with a literal robot and that he needs to stay focused on their work, but he doesn’t. He just tried to gently steer Jim away from a relationship that has no basis in reality. At the end of the episode, Spock tried to insist that he enter a room by himself and tries to block Jim from entering because he knows the truth will be revealed behind those doors, and the truth will completely crush Jim.
Spock loves Jim. He loves him so much that he places Jim’s feelings above logic and above his own preferences. He protects Jim in his own way from emotional as well as physical harm.
So much harder to not get jealous. Getting jealous would be easy! But true, he doesn’t. Spock’s actions have always been to protect Jim whenever Jim has gotten himself into a situation like this. It’s heart breaking hence to see him in episodes where Jim is not flirting but actually falls in love.
